Systems, methods, and devices for radiation beam alignment and radiation beam measurements using electronic portal imaging devices

ABSTRACT

Systems and methods for using electronic portal imaging devices (EPIDs) as absolute radiation beam measuring devices and as radiation beam alignment devices without implementation of elaborate and complex calibration procedures.

FIELD

The present disclosure relates generally to radiation therapy, and morespecifically to systems and methods for using electronic portal imagingdevices (EPIDs) as radiation beam measuring devices and as radiationbeam alignment devices without the need for implementing elaboratecalibration processes.

BACKGROUND

In radiosurgery or radiotherapy (collectively referred to as radiationtherapy) very intense and precisely collimated doses of radiation aredelivered to a target region (volume of tumorous tissue) in the body ofa patient in order to treat or destroy tumors or other lesions such asblood clots, cysts, aneurysms or inflammatory masses, for example. Thegoal of radiation therapy is to accurately deliver a prescribedradiation dose to the tumor/lesion and spare the surrounding healthytissue. The geometric accuracy of patient positioning relative to thetreatment beam, as well as the location and amount of dose delivered tothe patient is therefore important. There are a number of factors thatcould affect geometric and dose delivery accuracy, such as, incorrectpatient alignment relative to the treatment beam, misalignment of thelight field versus radiation field, shift of the skin marker, patientmovement, etc.

Because the radiation dose amount and dose placement need to besufficiently controlled for accurate patient treatment, the radiationtherapy machine itself needs to be properly tuned at the outset (on theproduction floor), and then continuously monitored through periodicchecks, such as, during initial installation or during routine usage ofthe machine by the customer, to ensure that the system is operatingwithin appropriate and expected parameters and standards.

Electronic portal imaging devices (EPIDs) were previously introduced toverify patient position. Thus, their primary use was for patientlocalization via portal imaging. However, due to their online efficiencyand data density, portal imagers/MV EPIDs have also received attentionas quality assurance (QA) devices. More recently, EPIDs have beenemployed for a variety of applications, including patient dosimetry andquality assurance (QA), to verify the treatment beams. Thus, the EPIDshave applications as imaging devices in machine-specific andpatient-specific quality assurance (QA) and commissioning andcalibration processes.

With the potential benefits of high data density and high resolution forEPID-based QA, there are also inherent problems associated with EPIDquality assurance (QA). For example, EPIDs are relative measurementdevices, convoluting the response variation due to radiation beam andper pixel characteristics (sensitivity, gain). Thus, the raw EPID imagescannot be used to assess radiation beam characteristics. Todifferentiate between contributions due to radiation beam and per pixelcharacteristics complex calibration procedures are required. Also thepixel characteristics may vary over time, requiring frequentrecalibration.

Currently existing EPID calibration processes try to correlate themeasurement of an absolute external measurement device, for example awater phantom, with the EPID image, thereby isolating the contributionsof beam and pixels. However, EPIDs are not dosimeters, as theinteractions of photons leading to an EPID image is different than theinteractions in water or tissue that lead to a radiation dose. Thus, theraw EPID image is not a dose image, and the EPID response deviates fromwhat would be expected based on water-based dose measurements. As such,direct correlation is not possible.

Thus, the ease of using EPIDs makes them attractive for qualityassurance (QA) applications, but the images must be corrected fornon-linear behavior of the electronics and inhomogeneous pixelsensitivities. Further, in order to use an EPID for measuring energychange, beam alignment, and beam tilt relative to the collimatorrotation axis, elaborate calibration procedures need to be implementedto calibrate the EPID's response to the measured values.

There is, thus, a need for an alternative approach to the extensivecalibrations procedures currently applied that is independent fromexternal dosimeters and from simulations, and a need for methods,systems, and devices by which EPIDs can be used as measurement devicesfor beam characteristics as well as for beam alignment without having toimplement elaborate calibration procedures.

Further, since many of the modern radiation treatment devices, such asmedical LINACS, are equipped with electronic portal imaging devices(EPIDs), there is a need for being able to use the EPIDs as beamalignment measuring devices without extensive calibration protocols inplace, in order to be able to perform automatic calibration, tuning, andverification of the radiation treatment systems and devices. Sincecurrently available radiation therapy machine tuning, calibration, andverification protocols are slow, inaccurate, require external hardware,and/or rely on subjective human decisions, employing EPIDs withoutcomplex calibration procedures, as disclosed throughout thespecification, reduces overall costs, processing, and analysis time, aswell as remove operator dependency.

SUMMARY

An object of the present disclosure is to provide a system and methodfor using an electronic portal imaging device (EPID) as a radiation beamcharacteristics measuring device as well as beam alignment measuringdevice without the need for extensive EPID calibration.

Another object of the present disclosure is to provide a system andmethod for measuring the number of converted high-energy photons usingan EPID without needing extensive EPID calibration.

Another object of the present disclosure is to provide a system andmethod for measuring photon flux and/or fluence using an EPID withoutneeding extensive calibration of the EPID.

Another object of the present disclosure is to provide imaging-basedmethods for verification of radiation treatment using an electronicportal imaging device (EPID) as a beam characteristics measuring deviceand beam alignment measuring device without the need for extensive EPIDcalibration.

Another object of the present disclosure is to provide systems andmethods for using an EPID device as a measuring device for determiningchanges in the radiation beam energy, symmetry, and flatness without theneed for complex EPID calibration.

Another object of the present disclosure is to provide a system andmethod for measuring radiation beam tilt using an EPID.

Another object of the present invention is to provide imaging-basedmethods for automatic calibration, tuning, and verification of radiationtreatment devices and systems. Since many of the modern radiationtreatment devices, such as medical LINACS, are equipped with anelectronic portal imaging device (EPID), the present invention providesmethods for using the EPID to perform the automatic calibration, tuning,and verification of the radiation treatment systems and devices, andtherefore, reduce overall costs, processing, and analysis time, as wellas remove operator dependency.

Another object of the present invention is to provide specificprocedures and image analysis algorithms for the automatic tuning,calibration, and verification protocols.

The present disclosure provides image-based quality assurance protocolsto verify that parameters and characteristics of a radiation treatmentdevice are within predetermined specifications using an EPID as a beammeasuring and beam alignment device without having to implement complexcalibration procedures.

The present disclosure also provides systems and methods for determiningradiation beam characteristics using an imaging device withoutcalibrating the imaging device response, the method comprising:acquiring one or more images using the imaging device, determining oneor more parameters from the one or more images, and determining one ormore characteristics of the radiation beam from the determined one ormore parameters. In embodiments, the one or more characteristicsincludes number of converted high-energy photons, photon flux and/orfluence, radiation beam energy change, radiation beam tilt relative to acollimator axis of rotation, radiation beam symmetry, radiation beamflatness, and radiation beam center change.

The present disclosure also provides radiation treatment systems,comprising: a radiation source configured to emit a radiation beam, animaging device configured to acquire one or more images, and aprocessing device configured to execute processor-executable processsteps for determining radiation beam characteristics withoutimplementing an imaging device response calibration protocol.

In embodiments, the process steps comprise: acquiring one or more imagesusing the imaging device, determining one or more parameters from theone or more images, and determining one or more characteristics of theradiation beam from the determined one or more parameters. Inembodiments, the one or more characteristics includes number ofconverted high-energy photons, photon flux and/or fluence, radiationbeam energy change, radiation beam tilt relative to a collimator axis ofrotation, radiation beam symmetry, radiation beam flatness, andradiation beam center change.

In embodiments, the imaging device is an electronic portal dose imagingdevice (EPID).

The present disclosure also provides for systems and methods forcalibrating the radiation treatment system based on the determined oneor more radiation beam characteristics. The calibrating can includecalibrating control elements of the radiation treatment system, thecontrol elements controlling the characteristics of the radiation beam.The control elements can include one or more of beam collimator devices,beam angle steering coils, beam position steering coils, shunt currentsources, beam flattening filters, beam scattering filters, dosimeters,gantry positioning devices, light sources, beam sources, and gun-cathodeheating controls.

The present disclosure also provides using an EPID as a measuring devicefor capturing various characteristics and parameters of a radiationtreatment device from images obtained using the EPID, analyzing thevarious characteristics and parameters from the EPID images, and usingthe information obtained from the images to modify the performance ofthe radiation therapy system to achieve the desired tuning andcalibration of the system.

The present disclosure also provides systems, devices, and methods forfast and less error prone tuning, calibration, and verification ofradiation therapy systems based on images obtained using electronicportal imaging devices, without the implementation of an EPID responsecalibration procedure.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments will hereinafter be described with reference to theaccompanying drawings, which have not necessarily been drawn to scale.Where applicable, some features may not be illustrated to assist in theillustration and description of underlying features.

FIG. 1 illustrates a radiation treatment system according to one or moreembodiments of the disclosed subject matter.

FIGS. 2A and 2B illustrate the rotation axes and coordinate frameorientation of the radiation treatment device of FIG. 1.

FIG. 3 illustrates a linac treatment head used in a radiation treatmentsystem operating in a photon generation mode.

FIG. 4 illustrates a linac treatment head used in a radiation treatmentsystem operating in an electron-beam generation mode.

FIGS. 5A-5C illustrate an exemplary imaging device used in the radiationtreatment device of FIG. 1.

FIG. 6 illustrates an exemplary flow diagram for measuring the number ofconverted photons using an EPID.

FIG. 7 illustrates another exemplary flow diagram for measuring thenumber of converted photons using an EPID.

FIG. 8 illustrates field edges and comb patterns formed by a collimatorused in the radiation treatment device of FIG. 1.

FIG. 9 illustrates an example of how the detected edges at differentcollimator angles are combined according to an embodiment.

FIG. 10 illustrates a radiation beam axis and collimator rotation axiswhen radiation source is neither shifted nor tilted.

FIG. 11A illustrates a radiation beam axis shifted relative to thecollimator rotation axis.

FIG. 11B illustrates radiation beam axis tilted relative to thecollimator rotation axis.

FIG. 12 illustrates an exemplary radiation beam profile generated by aradiation beam.

FIG. 13 illustrates an example flow diagram for measuring radiation beamtilt using an EPID.

FIG. 14 illustrates an example flow diagram for measuring energy changesusing an EPID.

FIG. 15 illustrates an example flow diagram for measuring beam flatnessand beam symmetry changes using an EPID.

FIG. 16 illustrates an example flow diagram for a calibration processusing an EPID.

FIGS. 17 and 18 illustrate example flow diagrams for implementationsaccording to embodiments.

FIG. 19 illustrates an example for graphical indicators for bolt turnsto correct misalignment according to an embodiment.

DETAILED DESCRIPTION

Patients undergoing radiation therapy are typically placed on thetreatment platform of a radiation treatment gantry. The radiation beamirradiates a region of interest in the patient, such as a diseased issueincluding a tumor or cancerous growth site. When delivering theradiation, a plurality of radiation beams may be directed to the targetarea of interest from several positions outside the body. The gantryincluding a radiation source can be rotated to provide the radiationbeams from different positions.

The ability to deliver the correct radiation dose to the target areadepends on several factors, including exact dose calibration, accuratelydetermined depth dose and off-axis dose characteristics, and knowledgeof the precise patient geometry used during irradiation. The influencerof these various factors depends on the type of radiation treatmentdevice used. For example, using isocentric treatment requiresunderstanding of the exact geometry in which the patient is treated.Therefore, the factors influencing accuracy of radiation beam deliveryis dependent on the mechanical precision and movements of the machineitself and of the machine and/or treatment accessories such as wedges,blocks, etc. As a result, a quality assurance protocol needs to beimplemented to test the dosimetric characteristics and the mechanicaland geometric integrity of the radiation treatment system.

There are numerous parameters, such as, beam alignment, beam symmetry,beam shape, beam energy, and beam flatness, associated with a radiationtherapy system that influence the accuracy of the radiation dosedelivered to the patient. Because these parameters depend on theaccurate alignment and placement of various mechanical elements/piecesof the radiation therapy system, the mechanical elements need to bechecked and tuned prior to the radiation treatment device beinginstalled and/or used in the radiation treatment facility. Because themechanical elements affecting these parameters tend to move, theparameters need to be regularly checked and, if a shift is observed fromtheir nominal preset values, the mechanical elements need to be adjustedand retuned during installment, and verified during regular preventivemaintenance inspection.

EPIDs have been used for evaluating parameters of the radiation therapysystem for some time. Generally, images obtained using an EPID arecompared with previously obtained images, and the discrepancies betweenthe images are associated with the parameters of the system. The ease ofusing EPIDs make them attractive for dosimetry applications, but theimages must be corrected for non-linear behavior of the electronics,inhomogeneous pixel sensitivities, scattering in the detector, and theEPID panel's complex energy response.

EPIDs have been used as relative dose measurement devices and asabsolute dose measuring devices. The relative dose measurements have thedisadvantage of requiring an external reference measurement of somesort, and the corresponding calibration schemes are often tedious. Theabsolute dose measurements on the other hand have the disadvantage ofrequiring complex and time-consuming calibration techniques to correctfor non-linearity of the EPID response. These calibration techniquesalso require accurate motion control of the EPID.

In either case, before an EPID can be used to detect dose, and thusnumber of detected X-ray photons per pixel, the EPID pixel responseneeds to be corrected for the inherent differences in response or gainof the individual pixels in the imaging matrix of the EPID. Measuringpixel sensitivity variation, however involves a complex process.

In the present embodiments, systems, methods, and algorithms aredescribed by which an EPID can be used to measure the number of detected(converted) X-ray photons, photon fluence, and photon flux,independently of the pixel gains, and thus without extensive calibrationof the EPID. This is done by measuring pixel noise across a series ofimages, and determining the number of detected X-ray photons per pixelbased on the measured mean pixel value and the calculated standarddeviation for every pixel i in a series of images. In alternativeembodiments, instead of measuring the pixel noise using a series ofimages, the local (spatial) noise within an image is measured and used,as described in detail throughout the disclosure.

In the present disclosure, systems and methods are described for usingthe EPID as a radiation characteristics and parameters measuring device,without having to implement elaborate calibration procedures. Forexample, the radiation beam tilt can be determined using an EPID withouthaving to calibrate the EPID for pixel sensitivity variations, becausethe tilt determination method is independent of the pixel gain. Themethod comprises determining the intersection of the beam axis with theimager by calculating the ratio of the pixel values of pixels i at twoenergies E₁ and E₂. The ratio depicts a circular beam profile centeredat the intersection of the beam axis with the imager, and is independentof the imager pixel gain. When the beam is tilted, a non-zero angle α isformed between the beam axis and the collimator rotation axis. Thisresults in the shift of the center of the beam shape as well as a slightdistortion of the shape, making it slightly elliptical.

The present invention provides systems and methods for evaluating aplurality of parameters of the radiation therapy system using anelectronic portal imaging device (EPID). The present invention alsoprovides systems and methods for verifying the parameters of theradiation therapy system using an electronic portal imaging device(EPID) without needing extensive calibration.

The present invention provides systems and methods for using an EPID asa measurement device for beam alignment, which outperforms the accuracyof the currently used gold standard (i.e., water phantom) by an order ofmagnitude.

The present invention also provides systems and methods for using anEPID as a measuring device for evaluation of radiation beam energychanges, beam flatness and beam symmetry changes without the need forcomplex calibration procedures. In embodiments, radiation beam energychanges, beam flatness, and beam symmetry can be determined using anEPID without having to calibrate the EPID for pixel sensitivityvariations, and thus, independently of the pixel gain, as described indetail throughout the disclosure. Using the EPID as described offsetsany variations in the pixel response of the EPID, and therefore, theEPID can be used without having to calibrate it for such variations. Thepresent disclosure also provides using an EPID as a measuring device forcapturing various characteristics and parameters of a radiationtreatment device from images obtained using the EPID, analyzing thevarious characteristics and parameters from the EPID images, and usingthe information obtained from the images to modify the performance ofthe radiation therapy system to achieve the desired tuning andcalibration of the system.

An exemplary radiation therapy treatment system which uses an EPID as ameasuring device is illustrated in FIG. 1. The treatment system 100 isconfigured to deliver radiation treatment to a patient 101. Thetreatment system 100 can be configured for dual-mode stereotactic orradiation therapy application, namely, the system 100 can be configuredto provide photon-based or electron-beam based radiation treatment to apatient 101 positioned on a treatment couch 102. The gantry 106 can be aring gantry (i.e., it extends through a full 360° arc to create acomplete ring or circle), but other types of mounting arrangements mayalso be employed. For example, a static beam, or a C-type, partial ringgantry, or robotic arm can be used. Any other framework capable ofpositioning the treatment beam source at various rotational and/or axialpositions relative to the patient 101 may also be used. The system 100also includes a treatment couch 102 which can be positioned adjacent tothe gantry 106 to place the patient 101 and the target volume within therange of operation of the treatment beam during radiation treatment. Thetreatment couch 102 may be connected to the rotatable gantry 106 via acommunications network and is capable of translating in multiple planesto reposition the patient 101 and the target volume. The treatment couch102 can have three or more degrees of freedom.

The radiation therapy system 100 includes a radiation treatment device103, such as, but not limited to, a dual-mode (photon and electron-beam)medical LINAC device configured for stereotactic or radiation therapyapplication. The radiotherapy device 103 includes a base or supportstructure 104 supporting the gantry 106. The gantry 106 is supporting anelectron beam accelerator module 108 which can include an electron gun114 for generating electron beams and an accelerator waveguide 115 foraccelerating the electron beams from the electron gun 114 toward anX-ray target 118 (when the radiation treatment device 103 operates in aphoton mode) or toward an electron beam exit window (not shown), whenthe radiation treatment device 103 operates in an electron-beam mode.The electron beam exit window allows the electron beam to exit theelectron beam accelerator module 108 and enter a LINAC treatment head110. The accelerating waveguide 115 can be mounted parallel to thegantry rotation axis, and thus the accelerated electron beam must bebent for it to strike the X-ray target 118 (when device 103 operates inthe photon mode) or the exit window (when device 103 operates in anelectron-beam mode). The accelerating waveguide 115 can also be mountedparallel to the collimator rotation axis. An electron beam transportsystem 116 can include bending magnets, steering coils, trim coils, anda gun cathode heating circuit can be used for bending and steering theaccelerated electron beams toward the X-ray target 118 or the exitwindow. The electron beam transport system 116 can bend an electron beamat 90 degrees, 270 degrees (achromatic bending) and at 112.5 degrees(slalom bending) by adjusting the shunt current applied to the bendmagnet from a current source (not shown). When the electron pencil beamhits the X-ray target 118, it generates the clinical photon beams(X-rays). The location at which the X-rays are generated is referred toas the radiation beam spot or radiation source.

In operation, electrons originating in the electron gun 114 areaccelerated in the accelerating waveguide 115 to the desired kineticenergy and then brought, in the form of a pencil electron beam, throughthe beam accelerator module 108 into the LINAC treatment head 110, wherethe clinical photons, such as X-rays, (when the device 103 operates inthe photon mode) or the electron beams (when device 103 operates in theelectron-beam mode) are produced. The LINAC treatment head 110 containsseveral components that influence the production, shaping, localizing,and monitoring of the clinical photon beams, as shown in detail in FIG.3, or the clinical electron beams, as shown in detail in FIG. 4.

The radiation treatment device 103 also includes a holding structure113, which could be a retractable robotic, servo controlled arm, holdingan imager 112 for acquiring digital images. The imager 112 is anelectronic portal imaging device (EPID). The holding structure 113 isused to position the EPID 112 and allow movement of the EPID 112vertically (along the Z-axis), laterally (along the X-axis), andlongitudinally (along the Y-axis). The EPID 112 can be mounted onto therotating gantry 106 in opposition to the radiation source, such that theclinical radiation beam, namely the photon or the electron beam, fromthe LINAC head 110 is received by the EPID 112. The EPID 112 can have adetector surface corresponding to the cross-sectional area of theclinical radiation beam.

In operation, the EPID 112 produces electronic signals providingmeasurements of the dose of the radiation received at the detectorsurface at regularly spaced positions over the detector surface. Thesignals from the EPID 112 are transmitted to a computer processor of thecontroller 120 where it is converted into a matrix of digital values,the values indicating the dose of radiation at each point of the imagersurface. A projection image derived from the matrix of digital valuescan be displayed on a display of the controller 120.

The controller 120 manages images and related information, such astransforming the data stream from the EPID 112 into a standard videoformat, the synchronization of the imager 112 and the LINAC treatmenthead 110 based on the different types of images acquired with the EPID112, as well as image transfer, frame processing, and image calibration.The controller 120 can also store and display the final dose image aswell as instructions for taking corrective actions. Controller 120 caninclude a computer with typical hardware, such as a processor, and anoperating system for running various software programs and/orcommunication applications. The computer can include software programsthat operate to communicate with the radiation treatment device 103,which software programs are operable to receive data from externalsoftware programs and hardware. The computer can also include anysuitable input/output devices adapted to be accessed by medicalpersonnel, as well as input/output (I/O) interfaces, storage devices,memory, keyboard, mouse, monitor, printers, scanner, etc. The computercan also be networked with other computers and radiation therapysystems. Both the radiation therapy device 103 and the controller 120can communicate with a network as well as a database and servers. Thecontroller 120 can also be configured to transfer medical image relateddata between different pieces of medical equipment.

The system 100 also includes a plurality of modules containingprogrammed instructions (e.g., as part of controller 120, or as separatemodules within system 100, or integrated into other components of system100), which instructions cause system 100 to perform different tuning,calibration, and verification functions related to the radiationtreatment device 103, as discussed herein, when executed. The modulescan be written in C or C++ programming languages, for example. Computerprogram code for carrying out operations as described herein may also bewritten in other programming languages.

The system 100 including the EPID 112 integrated with the radiationtreatment device 103 allows all image guidance activities, such as,image acquisition, image registration, image interpretation, EPID imagecalibration, and machine calibration to occur automatically andremotely. System 100 also allows capture of all data needed for theimage acquisition, evaluation, and calibration (i.e., data relating togantry, collimator jaws, MLC, light field source, EPID, EPID armstructure, phantom, filters, scattering foils, X-ray target, dosemeasuring device, beam steering coils, type of image to be acquired,EPID image calibration, etc.). Image interpretation to determine andevaluate different parameters and characteristics of the radiationtreatment device 103 can be performed using different algorithms. Thedetermination of adjustments needed to be made in the control elementoutputs based on the evaluated parameters and characteristics may alsobe determined using different algorithms. Once the required adjustmentsare determined, the necessary tuning and/or calibration and/orverification protocols are automatically sent to the radiation treatmentdevice 103 and the control elements are automatically or manuallyadjusted until their outputs fall within accepted ranges. FIGS. 2A and2B illustrate the radiation beam central axis, the gantry rotation axis,the treatment couch rotation axis, the collimator rotation axis, and theisocenter of system 100.

FIG. 3 illustrates a LINAC treatment head 110 when the device 103operates in a photon mode. The LINAC treatment head 110 can include oneor more retractable X-ray targets 118 where clinical photon beams, suchas X-rays, are produced, none, one or more flattening filters (FF) 117,which can be mounted on a rotating carousel or sliding drawer for easeof mechanical positioning of the filters 117 into the X-ray beam, dualtransmission ionization chambers 119, a collimating device (i.e.,collimator) including primary collimators 111, adjustable secondarycollimators with two upper jaws 121 and two independent lower jaws 123,multileaf collimators (MLC) 125, and a field defining light source 130.

Primary collimators 111 define a maximum circular radiation field, whichis then further truncated with the adjustable secondary collimators(121, 123) to produce rectangular and square fields at the LINACisocenter. The primary collimator 111 defines the largest availablecircular field size and is a conical opening that can be machined into atungsten shielding block, for example, with the sides of the conicalopening projecting on to edges of the X-ray target 118 on one end of theblock, and on to the flattening filters 117 on the other end. Thethickness of the shielding block is usually designed to attenuate theaverage primary X-ray beam intensity to less than 0.1% of the initialvalue. Any other applicable material besides tungsten can also be used.

The secondary beam defining collimators include four blocks, two formingthe upper jaws 121 and two forming the lower jaws 123. They can providerectangular and square fields at the LINAC isocenter, with sides of theorder of few millimeters up to 40 cm. Alternatively, the jaws could beindependent asymmetric jaws to provide asymmetric fields, such as onehalf or three quarter blocked fields in which one or two beam edges arecoincident with the beam central axis. The optional multileafcollimators (MLC) 125 can be made of 120 movable leaves with 0.5 cmand/or 1.0 cm leaf width, for example. For each beam direction, anoptimized intensity profile is realized by sequential delivery ofvarious subfields with optimized shapes and weights. When using MLCs,from one subfield to the next, the leaves may move with the radiationbeam on (i.e., dynamic multi-leaf collimation (DMLC)) or with theradiation beam off (i.e., segmented multi-leaf collimation (SMLC)). Suchan MLC system can cover fields up to 40×40 cm², for example, and canrequire 120 individually computer controlled motors and controlcircuits. Miniature versions of the MLC can also be used. For example,miniature MLCs that project 1.5-6 mm leaf widths and up to 10×10 cm²fields at the LINAC isocenter, could also be used.

The ionization chamber 119 could be a dual transmission ionizationchamber used for monitoring the photon radiation beam output as well asthe radial and transverse beam flatness. The ionization chamber 119 actsas an internal dosimeter, and can be permanently imbedded into the LINACtreatment head 110 to continuously monitor the radiation beam output.The ionization chamber 119 could also be sealed to make its responseindependent of ambient temperature and pressure. The ionization chamber119 can include a primary and a secondary ionization chamber with theprimary chamber measuring monitor units (MUs). Typically, thesensitivity of the chamber electrometry circuitry is adjusted in such away that 1 MU corresponds to a dose of 1 cGy delivered in a water ofphantom at the depth of dose maximum on the central beam axis whenirradiated with a 10×10 cm² field at a source to surface distance (SSD)of 100 cm. Once the operator preset number of MUs has been reached, theprimary ionization chamber circuitry shuts the radiation treatmentdevice 103 down and terminates the dose delivery to the patient 101.Before a new irradiation is initiated, the MU display is reset to zero.

In addition to monitoring the primary dose in MUs, the ionizationchamber 119 can also monitor other operating parameters such as the beamenergy, flatness and symmetry. Measurements of all of these additionalparameters requires that the ionization chamber electrodes of theprimary and secondary chambers be divided into several sectors, with theresulting signals used in automatic feedback circuits to steer theelectron beam through the accelerating waveguide 115 and the beamtransport system 116 and onto the X-ray target 118 or scattering foils127, thereby ensuring consistent beam flatness and symmetry.

The LINAC treatment head 110 can also include a field defining lightsource 130 to provide a convenient visual method for correctlypositioning the patient 101 for treatment using reference marks. Thelight source 130 may be mounted inside the collimator and can bepositioned at the location of the X-ray target 118 by a rotatingcarousel or a sliding drawer assembly, or it may be positioned to oneside of the collimator axis of rotation with the light reflected by amirror. In clinical operations, the light field illuminates an area thatcoincides with the radiation treatment field on the patient's skin andthe alignment of the light field with the skin marks on the patient isused as the final confirmation that the patient 101 is correctlypositioned with respect to the radiation beam. It is therefore importantthat the light field agrees (is congruent) with the radiation field.

When the radiation treatment device 103 operates in an electron-beammode, the LINAC treatment head 110 does not need the X-ray target 118and the flattening filters 117. FIG. 4 illustrates a LINAC treatmenthead 110 when the radiation treatment device 103 operates in theelectron-beam mode. To activate an electron-beam mode, both the X-raytarget 118 and the flattening filters 117 used in the photon mode areremoved from the electron pencil beam path. The electron pencil beamexits the beam accelerator module 108 through a thin window (not shown)usually made of beryllium, which minimizes the pencil beam scatteringand bremsstrahlung production. To produce clinical electron beams fromthe electron pencil beams, thin scattering foils 127 of a high atomicnumber (copper or lead, for example) are positioned into the electronpencil beam at the level of the flattening filters 117 in the X-raymode. In addition to the primary 111 and secondary collimators 121, 123,the clinical electron beams also rely on electron beam applicators(cones) 129 for beam collimation. The rest of the collimation and beamshaping elements are the same as in the photon-beam mode.

FIGS. 5A-5C illustrate an exemplary EPID 112 used to generate EPIDimages. The EPID 112 could be an amorphous silicon type detector panelincluding a 1 mm copper plate 201 to provide build-up and absorbscattered radiation, and a scintillating phosphor screen 202 made ofterbium doped gadolinium oxysulphide to convert the incident radiationto optical photons. The scintillating screen 202 can have a thickness of0.34 mm, for example. The EPID 112 can also include a pixel matrix 203created from an array of 1024×768 or 1280×1280 pixels i, where eachpixel i is made up of a photodiode to integrate the incoming light and athin film transistor (TFT) to act as a three-terminal switch forreadout. The EPID 112 can also include electronics to read out thecharge from the transistor and translate it into an image data.

The imager 112 can also be enclosed in a protective plastic cover 204with an air gap 205 between the protective cover and the copper plate201. Alternatively, layers of foam 206 and paper can be included betweenthe protective cover and the copper plate. The protective cover can beabout 3 cm above the effective point of measurement. The EPID can bepositioned at source to EPID distances (SDD) from 95 cm to 180 cm. Itcan also have an active imaging area of 40×30 cm² or 43×43 cm² (at anSDD of 150 cm), for example. The maximum frame acquisition rate can be15 frames/second, the permitted energy range can be between 4-25 MeV,and the permitted dose rates can be between 50-600 MU/min, for example.However, any other applicable EPIDs can be used, as the measuring device112.

EPID as X-Ray Photon Counting Device

Converting EPID images to dose generally includes generating acorrection matrix by which the EPID pixels need to be corrected. Thecorrection matrix takes into account corrections that need to be madefor inherent differences in response or gain of the individual pixels inthe imaging matrix of the EPID. Measuring pixel sensitivity variations,however, is a complex process. Thus, generating such a correction matrixgenerally involves a complex calibration process.

In the present embodiments, systems, methods, and algorithms aredescribed by which an EPID can be used to measure the number of detected(converted) X-ray photons without the need for complex calibrationprocedures. The methods disclosed allow for using an EPID to measure thenumber of photons per pixel without having to calibrate the EPID forpixel gain variations, because the number of detected X-ray photons perpixel are determined independent of the gain of the pixel. In thepresent embodiments, systems, methods, and algorithms are also describedby which an EPID is used to measure the photon flux and/or photonfluence without the implementation of complex calibration procedures.

Using an EPID as a photon measuring device in this fashion is based onthe observation that in an EPID, the image noise is dominated by X-rayphoton noise. Note that in a scintillator-based EPID, X-ray detection isbased on an indirect conversion, meaning that the X-rays are convertedinto visible light (subsequently also referred to as optical photons),which is then converted into electric charges. One single convertedX-ray photon generates a large number of optical photons. In contrast tothis, an EPID may also be based on direct-conversion detectors, in whichcase the X-ray photons are directly converted into electric charge. Inany case, the pixel value (i.e., signal measured for a pixel i in theEPID) is linearly related to the number of X-ray photons N_(i) detectedfor each pixel i.

The relationship between the pixel value and the number of detectedX-ray photons is:value_(pixel)=number_(photons)*gain  (1)or p _(i) =N _(i) *g _(i)  (2)where the gain g_(i) (in units of pixel values per photon) is a measureof how the digitally recorded signal relates to the actual photonsdetected, and number_(photons) refers to the number of detected X-rayphotons.

The mean (i.e., the average) of the pixel value m_(pi) over a series ofimages is related to the mean (i.e., the average) of the number ofphotons m_(Ni) by:mean_(value)=mean_(photons)*gain  (3)or m _(pi) =m _(Ni) *g _(i)  (4)

Similarly, the standard deviation σ (i.e., standard deviation is ameasure used to quantify the amount of variation or dispersion of a setof data values) and the variance (i.e., the average of the squareddifferences from the mean value) in a given pixel i is related to thestandard deviation and variance of the photon number by the sameproportionality constant g_(i):σ_(pi)=σ_(Ni) *g _(i)  (5)σ_(pi) ²=σ_(Ni) ² *g _(i) ²  (6)

The actual value of the variance σ_(pi) ² of the pixel signal is theresult of several noise variance contributions adding up, for instance,the noise of the detected x-ray photons, the noise of the visible(optical) photons, the dark (electronic) noise of the EPID pixel, andthe contribution coming from the variation of dose between individualimages.

In the particular case of megavoltage (MV) image series, and assumingthat an appropriate normalization is used to compensate dose variationsbetween individual images, the Poisson noise of the high-energy X-rayphotons is often dominant. In particular, the dark (or electronic) noiseof the EPID pixels is usually low enough to allow for a reliablemeasurement of photon noise, and in scintillator-based EPIDs, the noiseof the optical photons can usually be neglected.

Thus, it can be assumed that the noise is dominated by the Poisson noiseof the high-energy (i.e. X-ray) photons detected by the EPID. In thiscase, σ_(pi) is given by the square root of the number of X-ray photons,and the variance relates to the mean by:σ_(Ni) ² =m _(Ni)  (7)it follows that:m _(Ni) =m _(pi) /g _(i)  (8)σ_(Ni) ²=σ_(pi) ² /g _(i) ²  (9)and g _(i)=σ_(pi) ² /m _(pi)  (10)or N _(i)=(m _(pi)/σ_(pi))²  (11)

Thus, in an EPID as shown in FIG. 5B, the response of any pixel i in thepixel panel detecting N_(i) X-ray photons can be described as:p _(i) =g _(i) *N _(i)  (12)σ_(i) =g _(i) *√N _(i)(2)  (13)where p_(i) is the pixel value (i.e., the signal measured by pixel i),σ_(i) is the noise value of the pixel i (i.e., noise at pixel i), andg_(i) is the gain value of the pixel. By dividing equations (12) and(13), the number of detected X-ray photons per pixel N_(i) can beobtained from:N _(i)=(p _(i)/σ_(i))²  (14)

As such, by calculating the mean pixel value (corresponding to p_(i))and the standard deviation (corresponding to σ_(i)) for every pixel i ina series of images, the corresponding number of detected X-ray photonsper pixel N_(i) can be calculated independent of the gain g_(i) of thepixel.

Equation (14) was obtained assuming that the Poisson noise of the X-rayphotons is the only contribution to the noise variance σ_(Ni) ² inequation (7). However, the contribution of dark noise to the totalvariance becomes more important for lower photon number (e.g. lower doseper image) and will lead to a systematic underestimation of the X-rayphoton number obtained from equation (14). The influence of dark noisecan be taken into account by estimating the dark pixel noise variancefrom a series of dark images (i.e. taken without dose) and subtractingit from the total variance on the left side of equation (7).

To measure photon flux, process S100 as shown in FIG. 6 can beimplemented. In Step S101, a first EPID image is generated. For eachpixel i in the first image, the pixel value p_(i1), which represents thesignal measured at pixel i is obtained (S102). In step S103, a secondEPID image is acquired. As for the first image, for each pixel i in thesecond image, the pixel value p_(i2) is measured (S104). The process isrepeated for n number of images (S105), so that in the n^(th) image, foreach pixel i, the pixel value pin, is measured. Then, for each pixel i,the mean value p_(i) is calculated from the individual pixel valuesp_(i1), p_(i2), p_(i3), . . . , p_(in) (S107). The mean value p_(i) canbe calculated using:

$\begin{matrix}{p_{i} = \frac{{{pi}\; 1} + {{pi}\; 2} + \cdots + {pin}}{n}} & (15)\end{matrix}$

In Step S108, for each pixel i, the standard deviation σ_(i) iscalculated from the individual pixel values p_(i1), p_(i2), p_(i3), . .. , p_(in).

Alternatively, the pixel noise can also be calculated by averaging thesquare differences of pixel values of consecutive images and dividingthe result by 2, to be less sensitive to systematic long-term drifts.

Then, in step S109, for each pixel i, the number of photons per pixel iscalculated using:

$\begin{matrix}{N_{i} = \frac{pi}{\sigma\; i}} & (16)\end{matrix}$

The underlying assumption in determining photon flux and thus radiationdose this way is that the total number of created photons (the sum ofall N_(i)) is constant across the plurality of images. While this isgenerally not the case, it can be corrected by normalizing the pixelvalues by the dose per image. However, in order to do that, the dose perimage needs to be known more accurately than the pixel values, and thusit needs to have a signal to noise ratio (SNR) above √N, which wouldrequire an ion chamber accuracy of 0.3% or less. Moreover, for imageread-out synchronized to beam pulses, the dose per pulse in most casesis not even known.

In order to remove this issue, in an embodiment using Megavoltage (MV)image series, the mean pixel value within a region of interest (ROI) ofthe image itself is used for dose normalization. The mean pixel valuewithin an ROI of an image is the average of the pixel values of thepixels that comprise that ROI. For a ROI containing e.g. 100,000 pixels,this may result for instance in a signal to noise ratio (SNR) 100 timeslarger than for an individual pixel (for fully uncorrelated pixels, thesquare of the improvement factor would be 100,000). Thus, when the pixelvalues within the ROI across the series of images are used in thecalculations, it effectively replaces the ion chamber value used tonormalize the pixel values. If the dose normalization of an image seriesis based on the mean pixel value of an ROI, the resulting noisevariances of the individual pixels in this ROI will be slightly lowerthan for a dose normalization based on the ion chamber values of eachimage. The systematic error of dose normalization based on a mean ROIvalue, however, is by construction small compared to the individualpixel noise to be measured.

Thus, in alternative embodiments, instead of measuring the pixel noiseusing a series of images, the local (spatial) noise within an image ismeasured and used. However, this requires the measured beam to be moreor less constant within the region of interest (ROI) used to determinethe local noise, and the variation between individual pixel gains needsto be compensated by a pixel gain map that is precise enough to reducethe pixel gain variation to a level that is negligible compared to thelocal noise to be measured.

The process S200 for using an EPID as a photon flux measuring devicewhen local noise is measured is shown in FIG. 7, and includes thefollowing steps: In Step S201, a first EPID image is generated. Next, aregion of interest (ROI) is determined in the image (S202). For eachpixel i in the ROI, the pixel value pi, which represents the signalmeasured at pixel i is obtained in S203. By applying a previouslygenerated pixel gain map (S204), for each pixel i in the ROI, a pixelgain compensated pixel value is generated (S205) by scaling the pixelvalue p_(i) based on the pixel gain g_(i). The pixel gain map can begenerated using any known pixel gain map generation methods.

Then, the mean pixel value p of the gain compensated pixel values isdetermined in S206. The mean value p is calculated from the individualgain compensated pixel values p₁, p₂, p₃, . . . , p_(n) using:

$\begin{matrix}{p = \frac{{p\; 1} + {p\; 2} + \cdots + {pn}}{n}} & (17)\end{matrix}$

In Step S206, the local (spatial) noise value for the pixels in the ROIis also calculated by calculating the standard deviation a of theindividual gain compensated pixel values p₁, p₂, p₃, . . . , p_(n).

Then, in step S207, for each pixel i in the ROI, the number of photonsper pixel N_(i) is calculated using:

$\begin{matrix}{N_{i} = \frac{p}{\sigma}} & (18)\end{matrix}$

In all calculations, it is assumed that the pixel values are dark fieldoffset corrected pixel values.

EPID as Beam Alignment Measuring Device

For the accurate radiation delivery to the patient 101 under theradiation treatment device 103, it is important that the electron pencilbeam hits the X-ray target 118 at a perpendicular angle. When theelectron pencil beam hits the X-ray target 118 at a perpendicular angle,the radiation beam generated from the X-ray target 118 is symmetric. Thesymmetry of a radiation beam is considered with regards to the radiationbeam center as it is projected from the radiation source 118 past theradiation limiting devices (collimator jaws) to the isoplane.

The beam center is defined as the collimation rotation center at acertain height projected from the radiation source onto the image plane.The beam center can be calculated by taking a plurality of images (five,for example) with the collimator MLC rotating, and calculating the beamcenter from the set of (five) images obtained. Generally, the MLC jawsor leaves at a first height form a comb pattern, and the MLC jaws orleaves at a second, different height are used to shape the left/rightfield edges as shown in FIG. 8. In order to determine the beam center,the detected edges at different collimator angles are combined as shownin FIG. 9, for example. For each pair of subsequent edges, the anglebisector line is calculated. This results in four angle bisectors.Ideally, this set of lines intersects at the center of rotation. A leastsquares approach is then applied for finding the point in space with theleast distance to all bisection lines. This point in space is the beamcenter. Any other beam center determination method can be applied. Ifthe radiation source position is on the collimation rotation axis, thebeam center is independent of the height of the collimation element usedto determine the center. If not, the beam center is determined based onthe difference between the source position and the collimation rotationaxis.

In the radiation treatment field, the symmetry is considered along theX-axis and the Y-axis, with the Z axis being from the radiation sourceto the isoplane, and the Y axis increasing from the center toward thegantry stand structure, as shown in FIGS. 2A and 2B. Adjusting the angleof incidence of the electron pencil beam onto the X-ray target 118 canbe accomplished by adjusting the angle steering coils in the radial andtransverse directions, or with mechanical adjustments of the guide onlow energy radiation treatment devices.

If the electron beam does not hit the target 118 orthogonally, it willcreate an asymmetric radiation beam when looking at a flattened orun-flattened beam (i.e., no flattening filters 117 present). Dependingon the system, there are many other sources causing an asymmetricradiation beam. An asymmetric beam may introduce errors in the radiationbeam delivered onto the patient. Since the angle of incidence of theelectron pencil beam onto the X-ray target 118 is adjusted by adjustingthe angle steering coils in the radial and transverse directions, theangle steering coils of the radiation treatment device 103 arecalibrated in the radial and transverse angles so that the electronpencil beam hits the X-ray target 118 at a perpendicular angle, or theguide is mechanically adjusted. When the X-ray hits the target 118 at aperpendicular angle, the angle between the collimator rotation axis andthe radiation beam spot on the target is zero, as shown in FIG. 10. Ifthe angle between the radiation beam spot on the target and thecollimator rotation axis is not zero, the intersection of the beam axiswith the imager is offset, as shown in FIG. 11B. If it is determinedthat the beam is not properly aligned, a signal is sent to thecontroller 120 to automatically adjust the angle steering coils in theradial and transverse directions.

Determination of Radial and Transverse Beam Offsets

Using the EPID 112, the radial and transverse beam offsets can bemeasured by determining the beam center at two different elevations of acollimator using distal and proximal leaves and the difference betweenthe centers determined. If there is a difference between the twocenters, the difference is attributed to the radial and transversalradiation source offsets.

Determination of Beam Tilt

Generally, radiation tilt is determined indirectly by attributing theasymmetry of a beam profile to tilt, even though asymmetric beamprofiles can be due to beam shifts or beam tilts. The accuracy of thisindirect tilt measuring method is therefore not optimal.

In the present embodiment, a process S300 is disclosed by which the EPID112 is used to accurately measure tilt directly. Beam tilt can bedetermined using:

$\begin{matrix}{{\sin\;\alpha} = \frac{d}{SDD}} & (19)\end{matrix}$where d is the distance between the intersection of the beam axis withthe EPID and the projection of the source on the imager (along thecollimator rotation axis); SDD is the distance between the radiationsource and the EPID; and a is the angle between the beam axis and thecollimator rotation axis, as shown in FIG. 11B. When the beam is tilted,a non-zero angle α is formed between the beam axis and the collimatorrotation axis. In order to determine the tilt angle, the distance d thebetween the intersection of the beam axis with the imager and theprojection of the source on the imager needs to be determined.

1. Intersection of Beam Axis with Imager

For a perfectly aligned beam and a perfectly aligned imager, the pixelvalues p_(i) of the pixels i of the EPID show a circular beam shape B(r)as perceived by an EPID at location (x, y) on the EPID plane, if thepixel gain g_(i) is the same for all pixels, as shown in FIG. 12. Therelationship between the pixel values and the beam shape is expressedas:p _(i) =g _(i) B(r)  (20)with r being the radius of the circular beam.

The beam shape, however, depends on the electron energy E. Calculatingthe ratio q_(i) of the pixel values of pixels i at two energies E₁ andE₂ depicts a circular shape centered at the intersection of the beamaxis with the imager, independent of the pixel gains g_(i):

$\begin{matrix}{q_{i} = \frac{B\left( {r;E_{1}} \right)}{B\left( {r;E_{2}} \right)}} & (21)\end{matrix}$

When the beam is tilted, a non-zero angle α is formed between the beamaxis and the collimator rotation axis. This results in the shift of thecenter of the beam shape as well as a slight distortion of the shape,making it slightly elliptical.

Further, a slight tilt of the EPID with respect to the collimatorrotation axis also results in a slight scaling of the distance d, aswell as a slight distortion of the beam shape, making it slightlyelliptical. However, even if the EPID is tilted, a perfect beamalignment (i.e., an angle α equal to zero) still corresponds to adistance d equal to zero.

In order to determine the center of the elliptical beam shape, thecentroid of a circular shape representing the ROI pixels having a knowncenter is calculated using any of the applicable centroid calculationmethods. A radial weighting function can also be used during centroidcalculation to address ROI truncation artifacts. The distance betweenthe known center (projection of beam source center) and the centroid isthen minimized to find the center of the elliptical shape. Once thecenter of the elliptical shape is determined, the distance d can bedetermined by measuring the distance between the calculated center ofthe elliptical shape and the location of the projection of the beamsource on the EPID along the collimator rotation axis.

2. Calculation of Beam Tilt

Once d is known, the tilt angle can be determined by taking the inverseof the sine function:α=arc sin(d/SDD)  (22)

Accordingly, the beam tilt angle can be determined using an EPID byimplementing a process S300 as shown in FIG. 13 that changes theelectron energy and uses the ratio of two images to depict theintersection of the beam axis with the EPID. The process S300 is asfollows: In step S301, the EPID is positioned at a known distance SDDfrom the radiation source. The distance SDD could be about 150 cm, forexample. The EPID is then irradiated with a radiation beam having afirst energy E₁ to generate a first image, and with a second radiationbeam having a second energy E₂ to generate a second image (S302). Thesecond energy can be slightly different than the first energy, and thechange need not involve a change of the target and/or flatteningfilters. Then, for each pixel i, the ratio of the pixel value p_(i1) inthe first image and the pixel value p_(i2) in the second image iscalculated (S303). Next, a beam shape or beam ratio shape is generatedin S304 from the obtained pixel ratios. In S305, the center of thegenerated shape is determined using any applicable centroiddetermination methods. Once the center of the generated shape islocated, the distance between the location of the center of the shapeand the projection of the beam source on the EPID along the collimatorrotation axis, is measured (S305). From the measured distance d and theknown EPID to source distance (SDD), the tilt angle is calculated usingα=arc sin (d/SDD), where “arc sin” is the inverse of the sin function.

The angle α can also be translated into a photon fluence asymmetry Δ fora field size s at the isocenter using:

$\begin{matrix}{\Delta = {\frac{\left( {{SAD} + {s\;\sin\;\alpha}} \right)^{2}}{{SAD}^{2} - 1} \approx \frac{2s\;\sin\;\alpha}{SAD}}} & (23)\end{matrix}$

As disclosed, embodiments use the distance between the intersection ofthe beam axis with the EPID and the projection of the focal spot on theEPID to measure beam alignment using an EPID without having to implementcomplex calibration procedures, since the beam alignment determinationis made independent of the pixel gains.

Thus, according to described embodiments, the EPID can be used tomeasure directly the tilt of the radiation beam relative to thecollimator rotation axis without implementing complex calibrationprocedures. The EPID used this way is insensitive to mid-term andlong-term pixel gain changes, and it outperforms the accuracy of thecurrent gold standard (water phantom) by an order of magnitude.

EPID as Energy Change Measuring Device for (FFF) Beams

Currently, measuring energy using water phantom scans or specialphantoms (typically wedges) in combination with ion chamber arrays isstate of the art. However, energy measurements based on water phantomscans or ion chamber arrays require careful setup and alignment.Although EPIDs have also been used as relative measurement devices forbeam energy measurements, the EPIDs require elaborate calibrationprocedures. Also, the currently available calibration procedures do notinclude correlations with energy changes.

Embodiments described herein disclose systems and methods for usingEPIDs as measurement devices for beam energy changes, without requiringelaborate calibration procedures, since the beam energy changedetermination is made independent of the pixel gains. This is based onthe understanding that the flattening filter free (FFF) beam shape B asperceived by an EPID at location (x, y) on the EPID plane, off frompenumbra, can be approximated by a Gaussian function:B(x,y)=a·exp(−c·((x−x ₀)²+(y−y ₀)²))  (24)where a is the overall scaling parameter that depends on the beam outputwhich in turn depends on many different parameters, c is the curvatureparameter that depends on the electron energy of the beam, and x₀ and y₀are the coordinates of the beam center on the EPID plane and depend onthe electron beam position and tilt.

The actual pixel response p_(i) of a pixel i at location (x, y) on theEPID additionally depends on the pixel gain gi:p _(i) =g _(i) ·B(x,y)  (25)where B(x, y) provides the beam shape B as perceived by an EPID atlocation (x, y) on the EPID plane, (x, y) being the coordinates of pixeli on the EPID plane.

If B′ is the beam shape perceived by the same EPID under a differentbeam condition (e.g., with a different beam energy), then:B′(x,y)=a·exp(−c′·((x−x′ ₀)²+(y−y′ ₀)²))  (26)p _(i) ′=g _(i) ·B′(x,y)  (27)

Calculating ρ_(i) as the log of the ratio of the pixel values at thesedifferent beam conditions leads to:ρ_(i)=log(p _(i) /p _(i)′)=(c′−c)·(x ² +y ²)+d ₁ ·x+d ₂ ·y+d ₃  (28)

While the parameters d₁, d₂, and d₃ are functions of a, a′, c, c′, x₀,x₀′, y₀, and y₀′, the factor of the quadratic term (x²+y²), depends onlyon the difference of the curvature parameters (c′−c). Thus, thequadratic term (x²+y²) depends solely on the difference of the electronenergies of the two beam configurations. Therefore, by discarding d₁,d₂, and d₃ and correlating (c′−c) with the energy change, the energychange can be determined. The process S400 for performing energy changemeasurements of (FFF) beam is shown in FIG. 14 and includes thefollowing steps: In step S401, two EPID images are generated. In stepS402, for each pixel i in the first image, the pixel value p_(i) isdetermined by measuring the pixel signal, and for each pixel i in thesecond image, the pixel value p_(i)′ is determined by measuring thepixel signal. Next, for each pixel i, the log of the ratio of the pixelvalues ρ_(i) in the two images is calculated using ρ_(i)=log(p_(i)/p_(i)′) in S403. Since ρ_(i) is related to the change in energyas:ρ_(i)=log(p _(i) /p _(i)′)=(c′−c)·(x ² +y ²)+d ₁ ·x+d ₂ ·y+d ₃where (x²+y²) represents the position of the pixel in the EPID pixelpanel, substituting (c′−c) with do gives:ρ_(i) =d ₀·(x ² +y ²)+d ₁ ·x+d ₂ ·y+d ₃.  (29)Since (c′−c) depends on the electron energy change, d₀ also depends onthe energy change. Therefore, by determining d₀, the energy change canbe determined (S404). In order to do so, first a least-squares fit isperformed to find the values of d₀, d₁, d₂, and d₃ that provide the bestfit to:ρ_(i) =d ₀·(x ² +y ²)+d ₁ ·x+d ₂ ·y+d ₃over the pixels, where (x, y) are the coordinates of a pixel i on theEPID plane.

Least-squares fitting is a method of finding the best fitting curve to agiven set of points by minimizing the sum of the squares of the offsets(e.g., residuals) of the points from each of a set of candidate curves.An application of the least-squares fitting is curve fitting where acurve or a mathematical function is constructed as the best fit to aseries of data points, possibly subject to constraints. Curve fittingmay be performed through any known methods used in statistical packagessuch as R and numerical software, for example.

In order to estimate the values d₀, d₁, d₂, and d₃ to provide the bestfit to:

ρ_(i)=d₀·(x²+y²)+d₁·x+d₂·y+d₃, a curve fitting is performed for thegiven set of pixels i, and ρ_(i) value for each pixel i at location (x,y). As the relation is linear with respect to the values d₀, d₁, d₂, andd₃, any linear regression approach can be applied. Alternatively anynon-linear and/or iterative algorithm, for example theLevenberg-Marquardt algorithm, can be used. This can be done by startingwith initial values for d₀, d₁, d₂, and d₃ which values may be chosenbased on previously performed calibration tests. Then, at eachiteration, and for each pixel i, the parameter values d₀, d₁ d₂, and d₃are used to calculate the per pixel residual values using:R _(i) =p _(i) −d ₀·(x ² +y ²)+d ₁ ·x+d ₂ ·y+d ₃  (30)The sum R of the squares of the residual values R_(i) of all pixels i iscalculated next and compared to a previously obtained value of this sum.Subsequently, the parameter values d₀, d₁, d₂, and d₃ are adjusted toreduce R. For example, the Levenberg-Marquardt (L-M) algorithm may beused to adjust the parameter values d₀, d₁, d₂, and d₃ in the iterativeprocedure. This algorithm combines the Gauss-Newton method and thesteepest descent method, each of which could also alternatively be usedto adjust the parameter values d₀, d₁, d₂, and d₃. When the change in Rin two successive iterations is small enough (compared with a tolerancevalue), the fitting procedure is assumed to have converged. At thispoint d₁, d₂, and d₃ can be discarded. Any other best fit process canalso be applied. The adjusted parameter d₀ represents the parametervalue satisfying ρ_(i)=d₀·(x²+y²). Thus, the adjusted parameter valuerepresents (c′−c), which is dependent on the electron energy. As such,the adjusted parameter value can be correlated with the energy change.

In order to correlate the adjusted parameter value with the energychange, the ρ_(i) value is calculated over many pixels (several100,000). This typically allows determining changes of ρ_(i) in theorder of 0.001 per 10 cm field size. This corresponds to energy changesin the order of 10 keV. This energy change has been determined based onMonte Carlo simulations made using monoenergetic 6 MeV and 5.5. MeVelectron beams and a low-energy target on a 6 MeV magnetron-basedmachine, where amplitude changes in the order of 0.5% were obtained forthe high voltage power supply using the calculated value for ρ_(i).

In an alternative embodiment, the energy change determination method canalso be applied to flattened photon beams. The flattening filter (FF) inthis case is considered to be part of the EPID and implicitly includedin the pixel gains.

EPID as Beam Flatness and Beam Symmetry Change Measuring Device

The above described algorithm for energy change determination can alsobe used to determine beam flatness and symmetry change. For example,referring again to: ρ_(i)=log(p_(i)/p_(i)′)=(c′−c)·(x²+y²)+d₁·x+d₂·y+d₃

for a first beam with curvature c and a beam center at x₀ and y₀ and asecond beam with curvature c′ and a beam center at x₀′ and y₀′, theparameters d₁ and d₂ depend in principle on both electron energies(which affect the curvature parameters c and c′) and beam centers:d ₁=2c′x ₀′−2cx ₀  (31)d ₂=2c′y ₀′−2cy ₀  (32)

Accordingly, d₁ and d₂ are zero if the beam center does not changebetween the two beams (i.e., x₀=x₀′ and y₀=y₀′). Also, d₁ and d₂ areproportional to the beam center shift (i.e., (x₀′−x₀, y₀′−y₀)) if thebeam energy does not change (i.e. c=c′). Furthermore, for realisticenergy changes which are relatively small, d₁ and d₂ are predominantlyproportional to the beam center shift (i.e. (x₀′−x₀, y₀′−y₀)).Accordingly, beam flatness and beam symmetry changes can be determinedby implementing the following process S500 as shown in FIG. 15: In stepS501, two images are generated using the EPID. In each image, a regionof pixels is selected in the central part of the image (S502), theregion being off of the penumbra of for example an 18 cm×18 cm field.For example, each region can be a region including 512×512 pixelscorresponding to a field size of about 11 cm×11 cm. For each pixel i ineach region, the respective pixel values p_(i) and p_(i)′ are determinedin S502. Then, in step S503, for each EPID pixel i in the region, ρ_(i),which is the log of the ratio of the pixel values of pixel i in the twoimages is calculated using:ρ_(i)=log(p _(i) /p _(i)′),thus providing a ratio image of the two images. Changes in the beamflatness and beam symmetry are next calculated (S504) using thefollowing steps and algorithms: First, a least-squares fit is used tofind the values of d₀, d₁, d₂, and d₃ that provide the best fit to:ρ_(i) =d ₀·(x ² +y ²)+d ₁ ·x+d ₂ ·y+d ₃,over the pixels in the region of pixels, where (x, y) are the integralcoordinates of each pixel i on the EPID plane. Then parameter d₀ isconverted into a flatness change over a reference length L according to:

$\begin{matrix}{{\overset{\sim}{d}}_{0} = {{\exp\left( {d_{0} \cdot L^{2}} \right)}\mspace{14mu}{and}}} & (33) \\{\Delta_{Flatness} = {2 \cdot \frac{{\overset{\sim}{d}}_{0} - 1}{{\overset{\sim}{d}}_{0} + 1}}} & (34)\end{matrix}$and parameters d₁ and d₂ are converted into a symmetry change over areference length L according to:

$\begin{matrix}{{\overset{\sim}{d}}_{1} = {\exp\left( {d_{1} \cdot L} \right)}} & (35) \\{\Delta_{{Symmetry}\mspace{14mu} X} = {2 \cdot \frac{{\overset{\sim}{d}}_{1} - {1\text{/}{\overset{\sim}{d}}_{1}}}{{\overset{\sim}{d}}_{1} + {1\text{/}{\overset{\sim}{d}}_{1}}}}} & (36) \\{{\overset{\sim}{d}}_{2} = {\exp\left( {d_{2} \cdot L} \right)}} & (37) \\{\Delta_{{Symmetry}\mspace{14mu} Y} = {2 \cdot \frac{{\overset{\sim}{d}}_{2} - {1\text{/}{\overset{\sim}{d}}_{2}}}{{\overset{\sim}{d}}_{2} + {1\text{/}{\overset{\sim}{d}}_{2}}}}} & (38)\end{matrix}$Parameter d₃ is discarded.

The reference length L can be chosen to correspond to 5 cm at isocenter,for example, but any other length may be used. With the abovedefinitions, this leads to the following intuitive evaluation values,independent of machine parameters:

-   -   A flatness change of 1% corresponds to superimposing a change        that varies 1% from the center to the outside, over a diameter        of 10 cm.    -   A symmetry change of 1% corresponds to superimposing a change        that varies 1% from one edge to the other, over a length of 10        cm.

By means of a series of measurements, the flatness change can becorrelated with energy change. For example, a look up table or functionmay be derived based on the flatness changes for known energy changes(S505). Accordingly, the look up table or function may be subsequentlyused to estimate an unknown energy change from a derived flatnesschange.

By means of a series of measurements, the symmetry change can becorrelated with beam center change. For example, a look up table orfunction may be derived based on the symmetry changes for known beamcenter changes (S505). Accordingly, the look up table or function may besubsequently used to estimate an unknown beam center change for ameasured symmetry change. Assuming that either the shift or the tilt isconstant, the correlation can be translated to a change in the shift orthe tilt, as well.

System Calibration Using EPID

An exemplary automatic tuning/calibration process S600 by which thesystem 100 is tuned/calibrated with an EPID to operate within expectedparameters is shown in FIG. 16. The process S600 includes measuring,using an electronic portal imaging device (EPID) (S602), a plurality ofparameters/characteristics of the radiation therapy system 100 (S603),evaluating the measured parameters/characteristics against predeterminedstandards (S604), and tuning/calibrating the control elements of thesystem 100 (S605) based on the results of the evaluation so as to ensurethat the dosimetric characteristics and the mechanical and geometricintegrity of the radiation treatment device 103 is maintained. ProcessS600 includes steps which use the EPID to measure changes in the energyof the radiation beam, and radiation beam alignment (radial, transverse,and tilt) for the corresponding mechanical elementcalibration/tuning/adjustment. However, there are many more steps forfully tuning a system 100, including calibration/tuning/adjustment ofone or more of the: EPID's axis of motion, light source, collimatorjaws, steering coils, X-ray filters, bend magnet shunt current value,scattering foil, ionization chamber, and gantry, for example, as shownin FIGS. 17-18.

The calibration process S600 includes a plurality of calibration taskswhich could be fully or partially automatically performed using anelectronic portal imaging device EPID 112. The starting of thetuning/calibration process S600 can be initiated at the controller 120in Step S601, or via a second computer adapted to communicate withcontroller 120 to execute the calibration tests. In one embodiment theprocess S600 provides for an automated test sequence that quicklyacquires images and completes tests to help medical physicists determinethat a radiation therapy system is operating within specified parametersprior to treatment.

Using the EPID 112 in the process S600 allows for the determination ofbeam flatness and symmetry change with respect to a reference (e.g.,baseline). The determined discrepancies between the measured beamflatness and symmetry values and the baseline beam flatness and symmetryvalues could be used to adjust the angle steering coil accordingly.

The system calibration process S600 also includes measuring beam tiltand initiating the appropriate calibration of the beam if adetermination is made that there is a beam tilt relative to thecollimator axis of rotation. The process S600 provides the ability tomeasure radial and transversal source offset and tilt with gantry at anyposition. The calibration process S600 also provides the ability toreview completed measurements at any time, as well as visual indicatorsfor suggested alignment procedures including alignment bolts correctionturns, if the guide is mechanically adjusted, as shown in FIG. 19.Alternatively the angle steering could be adjusted accordingly.

Embodiments described herein therefore provide systems and methods wherean EPID can be used as a measurement device for measuring differentparameters of the radiation treatment system, without having toimplement a complex calibration of the EPID. The general process bywhich the radiation treatment system and device 103 is automaticallycalibrated using an electronic portal imaging device (EPID) includes thesteps of evaluating various parameters of the radiation treatment device103, followed by the automatic tuning of various elements of theradiation treatment device in response to the result of the evaluation.This can be achieved by taking one or more images using the EPID 112 byirradiating the EPID 112 with radiation beams (X-rays, electron-beams,etc.) from the LINAC treatment head 110. From the one or more images, aparameter of the radiation treatment device 103 is determined. Thisparameter can be any one of beam fluence or beam flux, beam symmetry,beam flatness, beam energy, beam linearity, beam dose, beam alignment,light field alignment, etc.

Then each parameter is evaluated to determine whether it falls within aprescribed range. If the parameter falls within a prescribed range, theprocess steps are repeated to determine and evaluate another parameterof the radiation treatment device 103. If the parameter does not fallwithin a prescribed range, the output of a control element of theradiation treatment device 103 affecting the respective parameter isadjusted until the parameter falls within the prescribed range. Theadjustment can include an adjustment in the radiation limiting(collimating) devices, the angle and position of the steering coils, thelocation of the flattening filters 117, the size of the bend magnetshunt current, the position of the scattering foils 127, the movement ofthe EPID arm support 113, the position and symmetry of the ionizationchamber 119, and the position of the light source 130, for example. Theadjustment can also be done manually, where appropriate. For example,manual adjustment of mechanical screws, bolts, or any other mechanicalpieces of the radiation treatment system can be manually done.

This calibration process can be automatically repeated until allparameters of the device are evaluated and the corresponding controlelement outputs adjusted. Any number of automatic routines using anydifferent type of feedback device can be inserted in the calibrationprocess with the same iterative tuning. When all the outputs are tunedand the parameters fall within prescribed ranges, the radiationtreatment device 103 is properly tuned, and the process stops.

FIGS. 17 and 18 illustrate examples of flow diagrams of how the variousimages, processing steps, and measurement values fit together.

It will be appreciated that the processes, systems, and sectionsdescribed above can be implemented in hardware, hardware programmed bysoftware, software instruction stored on a non-transitory computerreadable medium or a combination of the above. For example, a method forcan be implemented using a processor configured to execute a sequence ofprogrammed instructions stored on a non-transitory computer readablemedium. The processor can include, but not be limited to, a personalcomputer or workstation or other such computing system that includes aprocessor, microprocessor, microcontroller device, or is comprised ofcontrol logic including integrated circuits such as, for example, anApplication Specific Integrated Circuit (ASIC). The instructions can becompiled from source code instructions provided in accordance with aprogramming language such as Java, C++, C# or the like. The instructionscan also comprise code and data objects provided in accordance with, forexample, the Visual Basic™ language, LabVIEW, or another structured orobject-oriented programming language. The sequence of programmedinstructions and data associated therewith can be stored in anon-transitory computer-readable medium such as a computer memory orstorage device which may be any suitable memory apparatus, such as, butnot limited to read-only memory (ROM), programmable read-only memory(PROM), electrically erasable programmable read-only memory (EEPROM),random-access memory (RAM), flash memory, disk drive and the like.

Furthermore, the modules, processes, systems, and sections can beimplemented as a single processor or as a distributed processor.Further, it should be appreciated that the steps mentioned above may beperformed on a single or distributed processor (single and/ormulti-core). Also, the processes, modules, and sub-modules described inthe various figures of and for embodiments above may be distributedacross multiple computers or systems or may be co-located in a singleprocessor or system.

The modules, processors or systems described above can be implemented asa programmed general purpose computer, an electronic device programmedwith microcode, a hard-wired analog logic circuit, software stored on acomputer-readable medium or signal, an optical computing device, anetworked system of electronic and/or optical devices, a special purposecomputing device, an integrated circuit device, a semiconductor chip,and a software module or object stored on a computer-readable medium orsignal, for example.

Embodiments of the method and system (or their sub-components ormodules), may be implemented on a general-purpose computer, aspecial-purpose computer, a programmed microprocessor or microcontrollerand peripheral integrated circuit element, an ASIC or other integratedcircuit, a digital signal processor, a hardwired electronic or logiccircuit such as a discrete element circuit, a programmed logic circuitsuch as a programmable logic device (PLD), programmable logic array(PLA), field-programmable gate array (FPGA), programmable array logic(PAL) device, or the like. In general, any process capable ofimplementing the functions or steps described herein can be used toimplement embodiments of the method, system, or a computer programproduct (software program stored on a non-transitory computer readablemedium).

Furthermore, embodiments of the disclosed method, system, and computerprogram product may be readily implemented, fully or partially, insoftware using, for example, object or object-oriented softwaredevelopment environments that provide portable source code that can beused on a variety of computer platforms.

Alternatively, embodiments of the disclosed method, system, and computerprogram product can be implemented partially or fully in hardware using,for example, standard logic circuits or a very-large-scale integration(VLSI) design. Other hardware or software can be used to implementembodiments depending on the speed and/or efficiency requirements of thesystems, the particular function, and/or particular software or hardwaresystem, microprocessor, or microcomputer being utilized.

Features of the disclosed embodiments may be combined, rearranged,omitted, etc., within the scope of the invention to produce additionalembodiments. Furthermore, certain features may sometimes be used toadvantage without a corresponding use of other features.

It is thus apparent that there is provided in accordance with thepresent disclosure, systems, methods, devices, and algorithms for usingan EPID as a measuring device for photon count, photon flux, photonfluence, energy changes, beam tilt, beam flatness, and beam asymmetrydetermination without having to calibrate the EPID. It is thus alsoapparent that there is provided in accordance with the presentdisclosure, systems, methods, devices, and algorithms for using an EPIDas an imaging device for calibrating a radiation treatment systemwithout needing to implement extensive and complex calibrationprocedures.

Many alternatives, modifications, and variations are enabled by thepresent disclosure. While specific embodiments have been shown anddescribed in detail to illustrate the application of the principles ofthe present invention, it will be understood that the invention may beembodied otherwise without departing from such principles. Accordingly,Applicants intend to embrace all such alternatives, modifications,equivalents, and variations that are within the spirit and scope of thepresent invention.

The invention claimed is:
 1. A method of determining radiation beamcharacteristics using an imaging device, the method comprising:acquiring one or more images using the imaging device, the imagingdevice including a plurality of pixels; determining one or moreparameters from the one or more images; and determining one or morecharacteristics of the radiation beam from the determined one or moreparameters, wherein the one or more characteristics comprises number ofdetected photons, radiation beam energy change, radiation beam tiltrelative to a collimator axis of rotation, radiation beam symmetrychange, radiation beam flatness change, and radiation beam centerchange, wherein the determining of the one or more characteristics isindependent of variations in pixel gain values.
 2. The method of claim1, wherein determining the number of detected photons comprises:generating a plurality of images; measuring pixel values and one or morenoise values of a pixel across the plurality of images; calculating amean pixel value of the pixel from the measured pixel values; anddetermining a value corresponding to detected photons per pixel based onthe mean pixel value and the one or more noise values for the pixel. 3.The method of claim 2 wherein a value corresponding to detected photonsis determined for each pixel of the plurality of pixels.
 4. The methodof claim 2, wherein the value corresponding to detected photons perpixel is determined using: N=(p/σ)², where N is the number of photonsper pixel, p is the mean pixel value, and σ is a standard deviation ofthe pixel across the plurality of images.
 5. The method of claim 2,wherein the mean pixel value is determined for a plurality of pixelslocated in a region of interest (ROI) of an image.
 6. The method ofclaim 5, further comprising normalizing the pixel value using the meanpixel value of the ROI pixels.
 7. The method of claim 1, wherein theimaging device is an electronic portal dose imaging device (EPID). 8.The method of claim 1, wherein determining the number of detectedphotons comprises: determining a region of interest (ROI) in an image,the ROI including a plurality of pixels; determining gain compensatedpixel values for the plurality of pixels; measuring noise values for theplurality of pixels; determining a mean pixel value of the gaincompensated pixel values; determining a value corresponding to detectedphotons per pixel based on the mean pixel value and the noise values. 9.The method of claim 8, wherein the determining of the gain compensatedpixel values for the plurality of pixels includes determining a pixelgain map and correcting each pixel value of the plurality of pixelsusing the pixel gain map.
 10. The method of claim 8, wherein the valuecorresponding to detected photons per pixel is determined using:N=(p/σ)², where N is the number of photons per pixel, p is the meanpixel value of the gain compensated pixel values, and σ is a standarddeviation noise value for the pixels in the ROI.
 11. The method of claim1, wherein determining the beam energy change comprises: generating afirst and a second image; determine a first pixel value for a pixel bymeasuring pixel signal in the first image, and determine a second pixelvalue of the pixel by measuring a pixel signal in the second image;calculate a ratio of the first pixel value to the second pixel value;and determine a value of a parameter that provides a best fit betweenthe calculated ratio and a pixel location in a pixel panel of theimaging device, wherein the parameter relates to the beam energy change.12. The method of claim 11, wherein the best fit value is determinedusing a least-square fit algorithm.
 13. The method of claim 1, whereindetermining the beam flatness change and the beam symmetry changecomprises: generating a first and a second image; determine a firstpixel value for a pixel by measuring a pixel signal in the first image,and determine a second pixel value of the pixel by measuring a pixelsignal in the second image; calculate a ratio of the first pixel valueto the second pixel value; determine a plurality of parameter valuesthat provide a best fit between the calculated ratio and a pixellocation in a pixel panel of the imaging device; and determine the beamflatness change and the beam symmetry change based on the determinedparameter values.
 14. The method of claim 13, wherein a best fit valueis determined using a least-square fit algorithm applied to:ρ=d₀·(x²+y²)+d₁·x+d₂·y+d₃, where ρ is the log of the ratio, (x, y)represents the pixel location along the x and y imaging panel axis, andd₀, d₁, d₂, and d₃ are first, second, third and fourth parameters,respectively, wherein applying the least-square fit algorithm includesdiscarding d₃, the method further comprising converting a firstparameter value into flatness change, and converting the second andthird parameters into symmetry changes along the x and y axis.
 15. Themethod of claim 13, further comprising correlating the beam flatnesschange with the beam energy change, and the beam symmetry change withthe beam center change.
 16. The method of claim 15, further comprisinggenerating a look-up table including correlation values between the beamflatness change and the beam energy change, and between the beamsymmetry change and the beam center change.
 17. The method of claim 1,wherein determining the radiation beam tilt relative to the collimatoraxis of rotation comprises: using the imaging device positioned at apredetermined distance (SDD) from a radiation source, acquiring a firstimage at a first beam energy, and a second image at a second beamenergy; determining first pixel values by measuring pixel signals forpixels in the first image, and second pixel values by measuring pixelsignals for corresponding pixels in the second image; calculating ratiosof the first pixel values to the second pixel values; generating a ratioimage from the calculated ratio pixel values, the ratio image depictinga beam shape; determining a center of the beam shape; calculating adistance d between the center of the beam shape and a projection of theradiation source on the imaging device along the collimator axis ofrotation; and determining the radiation beam tilt based on thecalculated distance d and the predetermined distance (SDD).
 18. Themethod of claim 17, further comprising calculating the radiation beamtilt relative to a collimator axis of rotation includes calculating abeam tilt angle relative to the collimator axis of rotation using:${\sin\;\alpha} = {\frac{d}{SDD}.}$
 19. The method of claim 18, furthercomprising translating the calculated beam tilt angle into photonfluence symmetry.
 20. The method of claim 1, further includingcalibrating a radiation treatment device including the imaging devicebased on the determined one or more radiation beam characteristics.